Testing tiny wafers demands miniature moves from linear slides

In recent years, semiconductor labs have become billion-dollar enterprises. Accuracy in the automated testing of the wafers they produce is particularly critical and costly. In fact, for lower-cost products, testing can account for more than 25% of total fabrication expenses.

As an engineering director involved in automation design and development, Mark Filho recognized the need for accurate positioning in this industry, so he established Newmark Systems Inc., Rancho Santa Margarita, Calif. Its first customer — 15 years ago — needed a compact positioner for semiconductor wafer probing, an operation that occurs before wires are attached in the burn-in for final chip encapsulation. A probe checks the circuits and capacitance of dies or chips in the wafer, to verify whether the dies are functional and meet specifications. Next, the wafer is scribed and sliced into discrete semiconductors.

The future is miniaturization

The miniaturization of microprocessors was and is clearly the wave of the future, which in turn necessitates the miniaturization of fabricators and testers. Consider Newmark's NLS4 linear positioner, which runs from 2 to 24 in. and carries up to 25 lb on a 9-mm rail system. It was optimized for stability using FEA analysis, and incorporates miniature linear guides from NB Corp., Hanover Park, Ill., including two guides and four blocks.

When evaluating various miniature linear slide guides, Newmark chose the SEBS guide from NB due to its standard radial clearance, which is twice as accurate as other standard miniature guides the company considered. Most manufacturers don't claim that their preload eliminates all clearance, and their standards are expressed in plus-minus values. Negative values imply that there is some preload, so there is no gap — but positive clearance values imply that there's a possibility for detrimental spacing. In contrast, NB Corp. clearances are from zero to minus, making for greater accuracy. In short, the guide balls are larger than the guide spaces — eliminating clearance and adding pressure and rigidity.

Though there are instances for which no preload is desired — and designers trade accuracy and rigidity for reduced friction — high-accuracy applications such as wafer probing are an exception.

Guide variations abound: For the smallest and lightest loads, an extra-compact SEBS-BS (size 2) block is shorter than the standard. For applications requiring high rigidity, the SER guide uses crossed-roller bearings for more contact area than ball bearings. The SER is also available in all stainless steel and features non-retained rollers.

All NLS4 series stages are machined from 6061 aluminum alloys for weight reduction and stiffness. The drive uses a stainless steel ACME leadscrew with a zero-backlash plastic drive nut that automatically adjusts for wear.

Smaller still

Certain applications — from robotic pick-and-place to medical devices — require a greatly reduced footprint. For these, Newmark created a linear stage with one guide and bearing, instead of the usual two. This tiny positioner — the MSL — can lift 15 lb. and move at 3 to 4 in./sec; a lone linear guide bearing from NB Corp. supports the carriage over the MSL's entire 25 to 200 mm travel. What else? The MSL has two leadscrew pitches (2 and 10 mm), and comes standard with a size-17 stepper motor and end-of-travel limit switches. Like Newmark's other positioners, it is completely enclosed so that the guideways and screws are not visible — and its friction-free miniature guides show no signs of wear, even after one-and-a-half years of constant travel.